Since the discovery of the high temperature superconductors (perovskite type copper oxides), research of materials aiming a room temperature superconductor has been actively performed. As a result, a superconductor having a superconductive transition temperature (Tc) over 100K has been found.
There has been a progress in understanding of the formation mechanism of the superconductivity in the perovskite type copper oxides (for example, non-patent references 1 and 2.) On the other hand, compounds that include transition metal ions other than copper, or new compounds such as Sr2RuO4 (Tc=0.93 K) (Non-patent reference 3), Magnesium diboride (Tc=39 K) (Non-patent reference 4, Patent reference 1) and Na0.3CoO2.1.3H2O (Tc=5K) (Non-patent reference 5, Patent reference 2 and 3) have been newly found.
Strongly correlated electron system compounds having large interaction energy between conductive electrons compared to conduction band width, are known to have high possibilities to be superconductors having high superconductive transition temperatures. The strongly correlated electron system has been realized by layered compounds having transition metal ions at the skeletal structure. Most of such layered compounds belong to Mott-insulator, where antiferromagnetic interaction operates between electron spins in a way to align them antiparallel.
However, for example, in La2CuO4 which belongs to the perovskite type copper oxides, when Sr2+ ions are added at La3+ sites to form La2-xSrxCuO4, the itinerant electron state exhibiting metallic conduction is observed for x values within a range from 0.05 to 0.28, where superconductive state is observed at a low temperature and maximum Tc=40 K has been reported at x=0.15 (non-patent reference 6).
Recently, the inventors of the present application, found that a new strongly correlated electron compound having Fe as main component, LaOFeP and LaOFeAs can be superconductors, and applied as a patent (patent reference 4 and non-patent reference 7.) In the strongly correlated electron system, the itinerant electron state which exhibits metallic conduction is realized when a number of d-electrons takes a specific value, where transition to superconductive state occurs below a specific temperature (superconductivity transition temperature) when temperature is lowered. Further, the transition temperature of this superconductor varies from 5 K to 40 K, depending on numbers of conductive carriers. While in conventional superconductors such as Hg, Ge3Nb, the formation mechanism of the superconductivity has been attributed to the electron pair (Cooper pair) due to a thermal perturbation (BCS mechanism), in the strongly correlated electron system, the formation mechanism of the superconductivity has been attributed to the electron pair due to thermal perturbation of electron spins.
The inventors of the present application further found a superconductor comprising a strong electron correlation compound represented by LnTMOPn [here, Ln is at least one selected from a group consisting of Y and lanthanide elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), TM is at least one selected from a group consisting of transition metal elements (Fe, Ru, Os, Ni, Pd, Pt), Pn is at least one selected from a group of pnictogen elements (N, P, As, Sb)], and filed a patent application (Patent reference 5, Non-patent reference 8-10.)
The inventors of the present application also found a superconductor in compounds represented by A(TM)2(Pn)2 [here A is at least one selected from a group consisting of the second family elements in the long from periodic table, TM is at least one selected from a group of transition metal elements consisting of Fe, Ru, Os, Ni, Pd and Pt, and Pn is at least one selected from a group of the 15th family elements (pnictogen elements) in the long form periodic table], and filed a patent application (patent reference 6, non-patent reference 11.)